1. Technical Field
The present invention relates to a method for manufacturing a nitride semiconductor substrate for flattening a surface of a nitride semiconductor substrate.
2. Description of the Related Art
Generally, a processing method of flattening a surface of a workpiece such as a semiconductor wafer, includes a mechanical method of polishing a surface of a workpiece using abrasive grains, a polishing fluid (slurry) with abrasive grains dispersed therein, or a polishing cloth with abrasive grains fixed thereto; and a chemical method of etching the surface of the workpiece using a corrosive chemicals, etc., and a physical/chemical method for etching the surface of the workpiece using plasma.
In a case of a nitride semiconductor such as GaN (gallium nitride), Ga surface (Ga polar surface) for forming a device is hard and chemically stable. Therefore, there is no suitable chemical for etching the surface of a GaN substrate (GaN freestanding substrate) to be chemically flat at a sufficient etching speed, and therefore conventionally, the mechanical method or the plasma etching are performed for flattening the GaN substrate. (For example, see patent document 1). According to the mechanical method of patent document 1, when the GaN surface is processed into a mirror surface, diamond abrasive grains and SiC abrasive grains are used, to thereby obtain a surface with further excellent surface grain size, and as a final processing, slurry such as alumina and colloidal silica, etc., are used, to thereby obtain a flat surface.    Patent Document 1    Japanese Patent Laid Open Publication No. 2007-284283
However, in the polishing using the aforementioned mechanical method, mechanical scratch or damage is remained on the surface of the nitride semiconductor substrate such as GaN, and also there is a limit in a flattening/smoothing level of the surface. Further, the plasma etching involves a problem that a large-sized expensive apparatus is required, and the surface of the nitride semiconductor substrate is damaged by plasma.
An object of the present invention is to provide a method for manufacturing a nitride semiconductor substrate capable of manufacturing a nitride semiconductor substrate having a flat surface, without damaging the surface of the nitride semiconductor substrate.